All publications cited in this application are herein incorporated by reference.
The budding yeast Saccharomyces cerevisiae is routinely used in fermentations that produce biofuels and other high-value products. This yeast is capable of fermentation of hexoses (e.g., glucose) in the absence or presence of oxygen. Pyruvate is the theoretical endpoint of glycolysis, but continued fermentative metabolism reduces pyruvate to lactate or reduces the pyruvate derivative acetaldehyde to ethanol. The conversion to ethanol is typically favored in S. cerevisiae, although genetic modifications have yielded strains that have a metabolic endpoint of isobutanol or other commercially valuable products. In the presence of oxygen, pyruvate can alternatively be subjected to complete oxidation by enzymes of the tricarboxylic acid (TCA) cycle; the electrons stripped from pyruvate are ultimately donated to oxygen. Consequently, one outcome of glucose metabolism in yeast (and many other organisms) is the coupling of glycolysis (production of pyruvate) to the oxidative degradation of pyruvate with the ultimate transfer of electrons to oxygen (cellular respiration), resulting in the generation of ATP. If glucose is plentiful, S. cerevisiae metabolizes glucose largely (but not exclusively) by fermentation as it provides the most rapid way to gain sufficient energy for biosynthesis and cell growth. As glucose becomes limiting, cellular respiration is engaged to extract energy from alternative carbon sources including the materials that were the produced during the fermentative phase of growth.
Commercial strains of yeast have been engineered and selected to grow vigorously in preparatory phases of the industrial fermentation cycle. Most domestic fuel ethanol plants fermenting a corn feedstock purchase yeast in bulk from a wholesale supplier, often as a “dry active yeast” or “cream yeast”. In either case, preparations are produced on a large scale, beginning with small-scale “seed” propagation, and scaling to multi-thousand liter fermenters. In order to enhance biomass, the yeast are cultivated in a highly aerobic environment. Ultimately the yeast are harvested and prepared for sale and shipment to fuel ethanol plants. There, tens of kilograms of the purchased yeast stock are propagated aerobically using the corn mash to enhance the number of yeast cells that are added to the large (40,000 liter and larger) fermenters. This is the production phase of fermentation where anaerobic conditions induce the production of ethanol, isobutanol or other high-value bio-based chemicals.
The foregoing examples of related art and limitations related therewith are intended to be illustrative and not exclusive, and they do not imply any limitations on the inventions described herein. Other limitations of the related art will become apparent to those skilled in the art upon a reading of the specification and a study of the drawings.